When a transmitting device transmits data to a receiving device via a communication network, part of the data may be lost in the communication network (data loss occurs, e.g., packet loss may occur in a case that the communication network is the IP (Internet Protocol) network). Moreover, a delay time, which is a time before the receiving device receives data after the data is transmitted from the transmitting device, may become excessive.
Particularly in a case that the transmitting device transmits data representing multimedia that the real time property of video, audio or the like is high, when data loss occurs and/or when a delay time becomes excessive, the quality of experience relatively largely decreases.
Therefore, it is thought to be preferred to accurately estimate an available bandwidth, which is a communication bandwidth available in a communication path between the transmitting device and the receiving device, and properly determine a transmission rate based on the estimated available bandwidth. Here, a transmission rate is the amount of data transmitted by the transmitting device to the receiving device per unit time.
Accordingly, a technique for estimating an available bandwidth has been developed. As this type of techniques, “Pathload” described in Non-Patent Document 1, “pathChirp” described in Non-Patent Document 2, “IGI” described in Non-Patent Document, and so on are known.
An available bandwidth estimating device to which these techniques are applied is configured to estimate an available bandwidth based on an equation 1 and an equation 2. At first, this available bandwidth estimating device estimates an available bandwidth Ai(t) in a section i that is one of a plurality of sections configuring a communication path from the transmitting device to the receiving device, based on the equation 1.Ai(t)=Ci(1−ui)  [Equation 1]
The available bandwidth estimating device described above uses, as Ci×ui, an average value in a preset period of a communication bandwidth already used in the section i. Next, the available bandwidth estimating device estimates an available bandwidth A(t) in a communication path based on the equation 2.A(t)=min{Ai(t)}  [Equation 2]
Thus, the available bandwidth estimating device described above calculates a value as the result of subtracting an already used communication bandwidth from a physical bandwidth for each of the sections configuring the communication path, and estimates the minimum value of the calculated values as an available bandwidth for the communication path.    [Non-Patent Document 1] M. Jain, C. Dovrolis, “End-to-End Available Bandwidth: Measurement Methodology, Dynamics, and Relation with TCP Throughput,” Proc. SIGCOMM 2002, the Association for Computing Machinery, Vol. 32, pp. 295-308, October 2002    [Non-Patent Document 2] V. J. Ribeiro, R. H. Riedi, R. G. Baraniuk, J. Navratil, L. Cottrell, “pathChirp: Efficient Available Bandwidth Estimation for Network Paths,” Rice University, [searched on Dec. 3, 2009], Internet <URL: http://www.spin.rice.edu/Software/pathChirp/>    [Non-Patent Document 3] N. Hu, P. Steenkiste, “Evaluation and Characterization of Available Bandwidth Probing Techniques,” Proc. IEEE journal on selected areas in communications, the Institute of Electrical and Electronics Engineers, Inc., Vol. 21, No. 6, pp. 879-893, August 2003.
Data transmitted by a target device will be referred to as self-traffic. Moreover, data transmitted by another device by using a communication path that shares at least one section with a communication path from the target device to the receiving device will be referred to as cross traffic.
As a communication bandwidth used in a certain section increases, a data loss ratio and delay time relating to data flowing through the section increase. Therefore, in a case that the target device increases a transmission rate (a communication bandwidth to be used) relating to self-traffic, a data loss ratio and delay time relating to cross traffic increase.
On the other hand, in a case that the other device transmits data in compliance with TCP (Transmission Control Protocol), the other device is often configured to control to change the transmission rate depending on a data loss ratio, which is a ratio of occurrence of data loss, and/or a delay time.
Therefore, when the target device increases the transmission rate relating to self-traffic, the other device decreases the transmission rate relating to cross traffic. On the other hand, when the target device decreases the transmission rate relating to self-traffic, the other device increases the transmission rate relating to cross traffic. Thus, when the target device changes the transmission rate relating to self-traffic, the other device changes the transmission rate relating to cross traffic.
Therefore, the available bandwidth estimating device described above has a problem that it is impossible to estimate, with high accuracy, an available bandwidth when the transmission rate relating to self-traffic is changed.